Select all links from a Html table using XPath (and HtmlAgilityPack)

Question

What I am trying to achieve is to extract all links with a href attribute that starts with http://, https:// or /. These links lie within a table (tbody > tr > td etc) with a certain class. I thought I could specify just the the a element without the whole path to it but it does not seem to work. I get a NullReferenceException at the line that selects the links: